The term “stem cell” is a generic name for an undifferentiated type of body cell found in tissues of embryos, fetuses and adults, which has the potential of differentiating into a diverse range of specialized cell types. Stem cells are characterized by self-renewal, the ability to go through numerous cycles of cell division (while maintaining an undifferentiated state), and potency, the capacity to differentiate into specialized cell types in response to certain stimuli (environment), and even by plasticity, the ability to cross lineage barriers and adopt the expression profile and functional phenotypes of cells that are unique to other tissues.
Stem cells may be classified according to various criteria. Potency allows the classification of stem cells: pluripotent stem cells, multipotent stem cells and unipotent stem cells. Pluripotent stem cells have pluripotency to differentiate into any type of cells. Embryonic stem cells and induced pluripotent stem cells (iPS), which have recently received intensive attention from scientists, are representative of pluripotent stem cells. Adult stem cells show multipotency or unipotency. Among them are hematopoietic stem cells, mesenchymal stem cells, neural stem cells, etc.
In spite of various attempts to utilize the pluripotency of human embryonic stem cells in cell therapeutics, the high likelihood of oncogenesis and immune rejection response still remain and are difficult obstacles to overcome.
Induced pluripotent stem cells (iPS cells) have recently been suggested as a solution to these problems. iPS cells are a type of pluripotent stem cell artificially derived from a differentiated adult somatic cell by reprogramming. iPS cells may avoid the issue of immune rejection response because they are derived entirely from the patient, however, the risk of oncogenesis with iPS cells is still a problem to be solved.
As an alternative, mesenchymal stem cells are being promoted because they exhibit immunomodulatory effects and present no risk of oncogenesis. Mesenchymal stem cells are multipotent stem cells that can differentiate into a variety of cell types, including adipocytes, osteoblasts, chondrocytes, myoblasts, neuroblasts, myocardioblasts, hepatocytes, islet beta cells, vascular cells, etc., and are known to have the function of modulating immune responses.
Mesenchymal stem cells may be isolated from various tissues such as the bone marrow, umbilical cord blood, adipose tissue, etc., but are not sufficiently defined because cell surface markers are somewhat different from one another according to the origin from which the mesenchymal stem cells are derived. On the whole, if they can differentiate into osteoblasts, chondrocytes and myoblasts, have a spindle shaped morphology, and express the surface markers CD73(+), CD105(+), CD34(−) and CD45(−), the stem cells are defined as mesenchymal stem cells. In this context, mesenchymal stem cells of different genetic origins and/or backgrounds do not significantly differ from one another in terms of their definition, i.e., that of a mesenchymal stem cell, but are typically different from each other in terms of in vivo activity. Further, when mesenchymal stem cells are used as exogenous cell therapeutics, a limited pool of mesenchymal stem cells does not allow many choices or available options, even in spite of low in vivo activity.
In addition, the minimum number of mesenchymal stem cells necessary for them to be used as a cell therapeutic in regenerative medicine and/or cell therapy is approximately 1×109 cells. In practice, the minimum number is further increased in consideration of experiments for setting proper conditions and determining criteria. The supply of mesenchymal stem cells in such quantities from various origins requires at least ten in vitro passages. In this case, however, the cells become aged and deformed so that they may be unsuitable for use as cell therapeutics.
Thus, a culturing method effective for the mass production of mesenchymal stem cells is required.
Methods for culturing mesenchymal stem cells are described in Korean Patent Laid-Open Publication No. 2003-0069115, and literature [Pittinger M F et al. Science, 284: 143-7, 1999; Lazarus H M et al. Bone Marrow Transplant, 16: 557-64, 1995; and Kern et al., Stem Cells, 24: 1294-1301, 2006], but difficulties were found in guaranteeing the number of cells available for mass production. In addition, these methods suffer from the disadvantage of a decreasing number of mesenchymal stem cells in proliferative capacity every passage. For example, umbilical cord blood-derived mesenchymal stem cells cannot proliferate, but are rapidly aged after 9˜10 passages, and this phenomenon is found after 5˜6 passages in bone marrow- or lipid-derived mesenchymal stem cells. Therefore, there is a need for a novel method by which the number of mesenchymal stem cells can be increased to the extent sufficient for industrial applicability with higher simplicity and economical benefit compared to conventional methods.